The work aims to reveal cause and mechanism of corrosion resistance of 10CrMoAl steel. Seawater corrosion of 10CrMoAl steel was simulated by performing salt spray test (72 h, 168 h and 240 h). Corrosion rate, SEM, energy spectrum, XRD and electrochemical properties of the sample subject to test were analyzed and studied. After 72 h, 168 h and 240 h salt spray corrosion, the corrosion rate of the sample was 2.352 mm/a, 1.915 mm/a and 2.218 mm/a, respectively. After 72 h salt spray corrosion test, Cr and Mo were mainly enriched near the substrate; after 168 h salt spray corrosion test, Cr mainly congregated seriously on the rust layer close to outer surface; after the 240 h salt spray corrosion test, Cr and Mo congregated obviously on the substrate near the rust layer again, and "white light layer" appeared simultaneously. As corrosion time prolongs, variation trends of FeCr2O4 and γ-Fe2O3 content in the rust layer were consistent, that is, first rose continuously and then tended to be stabile. As corrosion time prolongs, corrosion rate first decreases and then increases, later decreases, and finally stabilizes. Enrichment law of Cr and Mo: first enrich at the junction of rust layer and substrate→corrosion resistant layer takes shape→corrosion layer peels off→Cr and Mo again enrich at the junction, this cycle goes round and round. This enrichment law has certain effect on corrosion prevention. In addition, FeCr2O4 exhibits significant corrosion resistance function when FeCr2O4 content is higher in rust layer.